Nancy, It will be difficult, The best chance will be to locate and wash with "oxcipital acid" this would be something a professional could locate. You might want to consider "camouflage" the stain with products a pro can apply for you.
If the product contained pendimethalin pre-emergent, you have a big problem. Pendimethalin (Halts, etc) is widely known to stain anything it contacts because the chemical itself is a dye. Sunlight doesn't fade it much on porous concrete. Get a scrub brush and some TSP cleaner and make a thin paste to scrub the area with. Follow with one of the automotive citrus hand cleaners (GOJO, etc).
Note that the label cautions against applying to stone, concrete, clothing or shoes.
One way is tri-sodium phosphate (TSP) which can be found at hardware and paint stores. Mix 1 ounce in a gallon of hot water. Wet the concrete and scrub vigorously. Let it soak into the concrete for 15 minutes then rinse well. Happy scrubbing!
believe he means exyalic acid - good source would be a granite tool supply house such as john r miles in elberton,,, oxyalic is common used to clean cemetery monuments,,, it will kill the grass but it will also grow back
We finally improved our rust on the driveway and sidewalk situation to the point when I can say it is resolved. We couldn't find the oxcipital (or exyalic) acid. We tried TSP to no avail. Finally, on the recommendation of a hardware store employee, we tried muriatic acid. We were very careful to follow the safety warnings so had no adverse effects. We applied this 3 times, each time increasing the acid to water ratio a bit. We put the mixture into a plastic watering can and gently spread
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it on the affected areas. Then we let it sit for a day and gently hosed it off, not getting it on the grass. After the third application, my husband used a stiff bristled broom to work on the worst spots. After hosing it down, it took a couple of days for the areas to fade almost completely. But they did, and we're happy.
Note that the label cautions against applying to stone, concrete, clothing or shoes.
Thanks to all who made suggestions.
Nancy D - Sarasota FL